Typically, a magnetic element, such as a magnetic memory element, includes a multi-layer thin film structure including ferromagnetic layers separated by a non-magnetic layer, hereinafter referred to as an insulating tunnel barrier, or barrier, layer. Information is stored in the magnetic layers as directions of magnetization vectors. Magnetic vectors in one magnetic layer, for instance, are magnetically fixed or pinned, while the magnetization direction of the other magnetic layer is free to switch in an applied field between the same and opposite directions that are called "parallel" and "antiparallel" states, respectively. In response to parallel and antiparallel states, the magnetic memory element represents two different resistances. The resistance has minimum and maximum values when the magnetization vectors of the two magnetic layers point in substantially the same and opposite directions, respectively. Accordingly, a detection of change in resistance allows a device, such as an MRAM device, to provide information stored in the magnetic memory element. The difference between the minimum and maximum resistance values, divided by the minimum resistance is known as the magnetoresistance ratio (MR).
Thin film structures of this type, more particularly magnetic tunnel junction elements, structurally include very thin layers, some of which are tens of angstroms thick. Of present concern is the fabrication of the insulating tunnel barrier, or barrier, layer formed between the ferromagnetic layers. The junction resistance varies approximately exponentially with the thickness of the barrier layer. This strong dependence on thickness makes it very difficult to produce devices with nearly the same resistance over typical substrate sizes used in manufacturing.
There are currently four common methods for forming insulating tunnel barrier layers made of aluminum oxide: a) natural oxidation of aluminum (Al) metal; b) ultraviolet (UV) assisted oxidation of aluminum (Al) metal; c) plasma oxidation of aluminum (Al) metal; and d) reactive sputtering of aluminum oxide. It should be understood that while aluminum oxide is disclosed as forming the insulative tunnel barrier, other materials which include insulative properties are anticipated by this disclosure, including, aluminum nitride, tantalum oxide, tantalum nitride, or the like.
The oldest method of forming an insulating tunnel barrier layer is through natural oxidation. Initially a layer of metal, such as aluminum (Al), is deposited on an electrode which is on some type of substrate. The aluminum is exposed to oxygen or air for a period of time, typically a number of hours. This exposure to oxygen or air causes the aluminum to oxidize and form an aluminum oxide layer. Subsequently the tunnel junction structure is completed by forming on top of the insulating layer the remaining electrode, ferromagnetic layers, or the like, dependent upon the type of device being formed.
One problem that exists with this type of device is that the initial depositing of the metal, here aluminum (Al), is non-uniform in thickness, and accordingly resultant differences in tunneling resistance across the layer occur. In addition, this method has been found to be very slow and there is limited flexibility in controlling the resistance of the formed insulating layer. As a result the resistance tends to be very low. Natural oxidation assisted by exposure to ultraviolet light is faster and potentially more controllable.
An alternative method for forming an insulating tunnel barrier layer, or insulating layer, is through plasma oxidation. During this process, aluminum is deposited onto a substrate and is subsequently oxidized with oxygen plasma, such as from an oxygen glow discharge, similar to neon plasma in a neon light. In general, this type of technique is hard to control and does not render a uniform thickness, thus resistance, across the substrate due to the lack of control exhibited over the oxygen discharge. Alternatively, an oxygen plasma source is utilized to oxidize the aluminum. This method provides for the control of resistance over a wide range and typically high magneto-resistance values are achieved. This method is generally similar to the previously described method utilizing a plasma discharge, yet more control can be exercised.
Yet another alternative method for forming an insulating tunnel barrier layer is reactive sputtering. During this process, aluminum is deposited, utilizing reactive sputtering, onto a substrate surface in an oxygen atmosphere. The aluminum reacts with the oxygen enclosed within the vacuum chamber to form aluminum oxide. Typically, results tend to exhibit high resistance and the end product is not uniform in thickness across the substrate leading to variations in resistance levels.
Other, less common, methods of producing insulating layers have been reported. A method similar to the plasma oxidation method is the oxygen beam method, deposition of an aluminum layer followed by exposure to an oxygen beam. The beam can be, for example, a low-energy oxygen ion beam or a low-energy atomic oxygen beam. A technique similar to reactive sputtering is sputter deposition from a compound target. This can be used with or without the addition of extra oxygen into the sputtering ambient, with or without an oxygen ion assist beam, or with or without an extra oxidation step after deposition.
Generally, while these four methods have been found to be useful for the fabrication of tunnel barrier insulating layers, uniformity of junction resistance over large wafer sizes used in semiconductor manufacturing, 150 mm to 300 mm diameter, has not been demonstrated. The resistance of the MTJ material, usually expressed as the resistance-area product (RA), has been shown to vary exponentially with both the metal layer thickness and oxidation dose for thickness and dose values that produce high MR. For example, a variation in aluminum thickness of only 5% can result in a variation in RA of over 30%. This strong dependence on the thickness and oxidation dose makes it very difficult to obtain high uniformity of RA. However, by adjusting the aluminum thickness profile and oxidation profile to offset each other, a resultant barrier layer with a small variation in thickness which is compensated for by a variation in the composition of the material provides for a uniform RA over the area of a substrate.
Accordingly, it is an object of the present invention to provide for a multi-layer thin film structure that includes an insulating layer having graded stoichiometry to compensate for the thickness profile and produce a uniform tunneling resistance across the insulating layer, more particularly, across the entire wafer structure as fabricated.
It is another object of the present invention to provide for a graded-stoichiometry insulating layer and method of fabricating the layer, for use in multi-layer thin film structures.
It is yet another purpose of the present invention to provide for a graded-stoichiometry insulating layer and method of fabricating the layer that includes precise control of the stoichiometry of the resultant layer, thus uniformity of device resistance across the substrate area.
It is another purpose of the present invention to provide for a graded-stoichiometry insulating layer and method of fabricating the layer that provides for uniformity of the resistance-area product in tunnel junction material.
It is still another purpose of the present invention to provide for a graded-stoichiometry insulating layer and method of fabricating the layer that includes the control of the lateral profile of the oxidation process, thus resistance of the resultant insulating tunnel barrier layer.
It is still a further purpose of the present invention to provide for a laterally-graded-stoichiometry insulating layer and method of fabricating the layer that is amenable to high throughput manufacturing.